Wouldn't it be great if you could easily adjust the output voltage of your
classroom VDG machine? By reducing the P.D. with respect to ground, you
could make it safe and non-scary for even the youngest preschooler. By
setting an appropriate value of potential, you wouldn't risk damaging
small instruments or nearby computers with the quarter-megavolt overkill.
And if you could rapidly alter the output as desired, you could perform
demonstrations which illustrate the effects of various values of e-field,
or even demonstrate varying e-fields.

The simple trick for adjusting your VDG voltage involves the creation
of a variable-gap gas-discharge voltage-regulator comprising a
small-diameter electrode adjacent to a flat-plate electrode. But since we
live our lives immersed in an appropriate gas, it's simple. The only
parts required for this are:

Piece of wire

Cable ties, or rubber band, or tape

Simply do as shown in the diagram. Connect a piece of wire to the
electrical ground
terminal on the base of your VDG machine. (If it has no ground terminal,
tape the bare end of the wire to the metal base of the generator.) Run
the wire up along
the column to where it barely touches the base of the sphere. Temporarily
fix it in place with plastic cable-ties (zip several together if yours
are
too short)
or use rubber bands, or simply tape it in place.

The voltage of your VDG machine can now be controlled by varying the
distance between the tip of the wire and the upper sphere terminal. When
the wire touches the sphere, the voltage of course is zero. If you pull
the wire slowly downwards, the gap ("D" in the diagram) increases slowly,
and the voltage on the sphere increases. To set the voltage to maximum,
move the tip of the wire down the column all the way to the base. To set
the voltage to approximately half-maximum, move the tip of the wire down
the column midway between the base and the sphere. It's not really
necessary to place the wire against the column. So, if you hold the
grounded wire in your hand and move the tip slowly towards and away from
the sphere, the output voltage will slowly decrease and increase.
Move it in and out, and you silently broadcast low frequency
electromagnetic sine waves out into space! :)

SUGGESTED USES

You can elmiminate any
accidental shocks to students during the "Hair Raising" demo.
First adjust the wire tip upwards for zero voltage. Run the
generator, let your victim place his/her hand on the sphere terminal,
then pull the wire down to increase the voltage. When your victim has
had enough fun, push the wire back up into contact with the sphere, which
leaves their body without an excess charge. He/she won't get zaps from
the floor, other students, etc., upon climbing down from the insulating
stool. And the next victim won't get a zap from terminal, since it was
left with zero excess charge at the end of the demo.

Eliminate accidental shocks to yourself by always holding a (bare
uninsulated)
voltage-adjustment wire during demonstrations, and position yourself so
the tip of the
wire is between you and the VDG sphere. This lets you easily raise and
lower the voltage, and when it comes time to turn off the VDG, you won't
get a nasty zap from the power switch if first you use the grounded wire
to adjust the sphere voltage to zero.

Tape a group of strips of paper tissue (2cm x 20cm) all over the
sphere-terminal
of your VDG machine. Set the voltage to zero, turn the machine on, then
slowly raise the voltage. The tissue strips will slowly stand on end.
Move the wire rapidly in and out, which varies the voltage,
which makes the tissue strips respond. Try this with soap bubbles. Try
this with homemade electrometers. How far can you "broadcast" these
waves of changing e-field?

Build the Pop-bottle motor detailed elsewhere on my website. Connect
it to your VDG, and use the voltage adjusting wire to set the motor speed
to any value desired. Or, if you are using your VDG to drive various
student-built electrostatic devices, you can use the voltage-adjusting
wire to
slowly raise the applied voltage (thus avoiding sudden discharges.)

Obtain a professional rotary-plate Electrometer or "field mill," then
practice your graph-making ability by calibrating the distance of the
wire-gap "D" against the actual measured output voltage of your VDG
machine.

End confusion over the speed control knob found on some VandeGraaff
machines. The speed control does not control the voltage, it controls the
speed of the belt, which controls the electric current going up the belt,
and so controls the rate of charging of the sphere terminal. A low
setting on the speed control will make the sphere terminal charge to
maximum voltage ...but more slowly. To really control the output voltage,
turn the speed
up medium-high, then move the grounded control-wire up and down.

Ambitious people with large VDG machines can even add a remote-control
voltage regulator in the form of an automobile retractable FM radio
antenna having a sharpened tip. Provide a 12V supply for the motor, and
an "up" and "down" button, and
you can control your big nasty machine from a safe distance.

HOW IT WORKS

If a sharply-pointed ground wire is held near the
sphere of an operating VandeGraaff machine, a tiny corona discharge or
"St. Elmo's Fire" will be created on the wire tip. This corona discharge
emits positive and negative air ions, which causes the immeditately
adjacent air to become
conductive. Ions pulled from the conductive air stream flow between the
tip of the ground wire and
the sphere, and it acts as a resistive "short circuit" path to ground,
which shorts out the VDG machine and drops the output voltage to a very
low level. But that's only part of it.

If the wire is held very very close to the sphere terminal, the
potential falls almost to zero. But why does the potential rise higher
for a
large gap between the wire and the sphere surface? I
don't have a ready answer for this. I suspect that it involves a
complicated
feedback process akin to the one which occurs within gas-discharge
voltage regulator tubes, and in Zener voltage regulator diodes.

But here's my attempted explanation anyway!
As the ground wire is pulled away from the sphere, the strength of the
e-field at the tip of the wire drops below a level which supports the
"avalanche" process which keeps the tiny plasma-flame ignited. The corona
discharge comes close to winking out, and its rate of ion emissions drop.
This increases the overall resistance of the invisible conductive path
between the wire and the sphere, which lets the sphere's potential rise
higher. The stronger e-field again increases the corona discharge at the
tip of the wire. The VDG machine as a whole is producing approximately a
constant current, so as the air-path resistance increases, the VDG output
potential increases. As the potential increases, the e-field at the wire
tip increases just enough to keep the same ion-current going within the
corona all the time. Or put more simply: as the distance between wire tip
and the VDG sphere is varied, the rate of corona discharge at the wire tip
remains constant, and the total current in the air path remains constant,
while the resistance of the air path and the potential on the sphere will
go up or down. See what happens with a variable resistor and constant
current? Ohm's law says that this gives variable PD.